Do not make symbols globally visible by default on *nix (when supported),
on Windows this is always the case. To prevent hidden errors (since most
developers seem to test on *nix), use -fvisibility=hidden when GCC is used
and make the MODULEAPI and CF_PLUGIN macros use __attribute__ to mark those
as visible. This will not break on any compiler not supporting this, but will
prevent future hidden errors of this type.

A further advantage with this is that it reduces risk of symbol name collision
between various dynamic objects.

plugins/cfpython/include/cfpython_proto.h, plugins/cfpython/cfpython.c: Fix
broken give command. Was broken due to MSW removing -Bsymbolic when building
plugins about half a year ago. Also change some other symbols to static in the
plugins to avoid more issues like this.
plugins/cflogger/include/cflogger_proto.h, plugins/cflogger/cflogger.c,
plugins/citylife/include/citylife_proto.h, plugins/citylife/citylife.c,
plugins/cfrhg/include/cfrhg_proto.h, plugins/cfrhg/cfrhg.c,
plugins/cfanim/include/cfanim.h, plugins/cfanim/include/cfanim_proto.h,
plugins/cfanim/cfanim.c, plugins/cfnewspaper/include/cfnewspaper_proto.h,
plugins/cfnewspaper/cfnewspaper.c: More changes in the same style as the above
python changes to reduce possible symbol lookup conflicts. Also make some
functions static.
plugins/cfrhg/include/cfrhg_proto.h, plugins/cfrhg/cfrhg.c: Fix a typo in cfrhg
(was "cfrgh" in one place).

Clean up in plugins, fix a lot of gcc warnings (in plugins), update prototype
headers in plugins, make some functions static. Also update build system for
cflogger to make it not include common plugin stuff in header. Followed example
set by cfnewspaper's Makefile.am. Make the template plugin include prototype
header. (It is a good idea to do so, for various reasons.) Also add some missing
newlines at end of copyright header comments in some files. Also make some
variables const (in python plugin) in order to fix gcc warnings and to make
code cleaner (a string literal is always a const char* in C90 or later, but
char* is allowed for compatibility reasons, though it will actually be a const).